The present invention relates to a traveling wave tube device and, more particularly, to a high power traveling wave tube device wherein a coupled cavity type delay circuit is used as a high frequency circuit unit and a periodic permanent magnet assembly is used as an electron beam focusing unit.
This type of traveling wave tube device comprises an electron gun unit to emit an electron beam, a high frequency circuit unit to amplify a high frequency wave through interaction with the electron beam, a collector unit to catch the electron beam, input/output coupling units to input and output high frequency power to the high frequency circuit unit and a beam focusing unit to focus the electron beam.
The high frequency circuit unit, a coupled cavity type delay circuit, is used for deriving out the high power output, and a periodic permanent magnet (PPM) assembly is used for the beam focusing unit in view of its compact size and its economical and practical advantages.
A known traveling wave tube device of the structure described is, for instance, disclosed in FIG. 31 and its legend appearing on page 58 of "DEVELOPMENT OF A HIGH POWER 12 GHZ PPM FOCUSED TRAVELING WAVE TUBE" published in May, 1975 by NASA Lewis Research Center, Contract NSA3-14391.
While said PPM assembly has the characterizing features explained above, it is defective in the following points. The PPM assembly comprises a plurality of ring shaped magnets disposed in the axial direction of the tube and a plurality of ring-shaped pole pieces interposed between the magnets. The magnets and the pole pieces should be placed as close to the electron beam as possible in order to cause the magnetic field generated by these magnets to act on the electron beam. The electron beam emitted from the electron gun unit should pass the high frequency circuit unit or the drift tubes at a high density in order to prevent the beam from scattering and damage to the high frequency circuit unit. For this purpose, the above mentioned PPM assembly must cover uniformly the whole distance of the high frequency circuit unit disposed between the electron gun unit and the collector unit. However, it is also absolutely necessary for the high frequency circuit unit to be provided with the means for input and output of the high frequency waves. On one hand, designers and manufacturers of the conventional type traveling wave tubes have taken into consideration the problems uniquely observed in the above mentioned PPM structure have offered the beam focusing unit, such as the PPM assembly, independently and separately from the high frequency circuit, such as the coupled cavity type delay circuit, and the input/output coupling units with a considerable characteristic allowances. These units are assembled in the final stage of production with other parts such as the electron gun unit, the collector unit, etc. Any characteristic dispersions are generally corrected by electrical and mechanical adjustments that are subsequently performed in the final stage.
In the conventional type traveling wave tube disclosed in the previously noted publication, portions of magnets or the pole pieces of the PPM assembly covering the high frequency circuit are omitted and waveguides in the input/output coupling unit are coupled to the high frequency circuit unit through the omitted portions. Such structure was quite sufficient for low power output traveling wave tubes. However, this structure was found insufficient high power output traveling wave tubes, for instance, 1 kw at 14 GHZ, as the phenomena to be described later became apparent as a result of the part omitted in the PPM assembly. In the above mentioned structure, the magnetic field intensity on the center axis within the drift tubes near the missing part in the high frequency circuit unit became smaller and eccentric, and the magnetic field distribution on the section crossing perpendicularly with the axis in the drift tubes became non-uniform and eccentric with regard to a certain dimension in the drift tubes. When such a situation appeared, the electron beam passing through the drift tubes became scattered or deviated from the center axis of the drift tubes and impinged upon the wall of the drift tube or its vicinity thereby melting and destroying the portion of the high frequency circuit unit.
In order to overcome such problems, it was proposed to enlarge the magnets near the omitted portion and compensate for the decrease in the magnetic field intensity on the nearby center axis, or to use a different magnetic material with a larger B-H product energy such as a rare earth metal. It is conceivable to correct the eccentric magnetism by leaking the magnetic field where it is intense by attaching a suitable ferromagnetic material to the outer periphery of the magnet.
The above methods, however, are mere extensions of the conventional design concept, and require a great deal of time and skill to arrange the magnetic field intensity of the PPM assembly at a predetermined level.